Learn what the latest research says

Kettlebells

Kettlebell training programs seem to be able to improve strength in (mostly untrained) individuals. They also seem able to improve proxies of power (vertical jump height or ballistic resistance training exercise performance) in similar populations.

Kettlebell training programs seem to display all the necessary characteristics for increasing aerobic fitness but evidence from long-term trials is conflicting.

Kettlebell swings may transfer well to sprint running, because they involve high levels of horizontal force as well as high levels of gluteus maximus and hamstrings muscle activity.

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CONTENTS

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SUMMARY

PURPOSE

This section provides the summary of research exploring whether kettlebells can develop strength, power, and aerobic fitness. It also covers key biomechanical features of kettlebells that suggest how kettlebell exercises might best transfer to athletic performance.

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SUMMARY EVIDENCE FOR KETTLEBELLS

The following table grades the amount of research available for the effects of kettlebell training on different outcomes in (mostly untrained) subjects using a 5-point scale (very small, small, moderate, large, very large). It also grades our confidence for the prevailing finding, if there is one using a 5-point scale (very low, low, moderate, high, very high).

Training outcome

Research quantity

Confidence

Long-term study findings

Strength

Small

Low

Strength increases with kettlebell training

Power

Small

Moderate

Proxies of power increase with kettlebell training

Aerobic fitness

Small

Very low

Aerobic fitness increases with kettlebell training

Transfer

Small

Very low

Kettlebell swings may transfer well to sprint running

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CONCLUSIONS REGARDING KETTLEBELLS

Kettlebell training programs seem to be able to improve strength in (mostly untrained) individuals. They also seem able to improve proxies of power (vertical jump height or ballistic resistance training exercise performance) in similar populations.

Kettlebell training programs seem to display all the necessary characteristics for increasing aerobic fitness but evidence from long-term trials is conflicting.

Kettlebell swings may transfer well to sprint running, because they involve high levels of horizontal force as well as high levels of gluteus maximus and hamstrings muscle activity.

BACKGROUND

PURPOSE

To provide a brief background to kettlebells and a history of their use, common exercises that have been described in the literature, and their potential programming applications.

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BACKGROUND

Introduction

Kettlebell training is essentially a form of resistance training, although most standard kettlebell training programs make use of predominantly ballistic exercises for relatively high numbers of repetitions and do not train to muscular failure. The use of kettlebells in the USA and Europe is a recent phenomenon, although they are thought to have a much longer heritage of use in Russia. Researchers have only just started to investigate the use of kettlebells in strength and conditioning programs over the last few years. Consequently, there is little literature on which to base conclusions.

Overall, the initial response from the strength and conditioning community was relatively negative in respect of using kettlebells (e.g. Chiu, 2007) on the basis that their low loads implied large limitations for athletes (40kg was the maximum kettlebell load available at that time). However, later research has demonstrated that the unique biomechanics of certain kettlebell exercises may transfer well to certain key sporting movements. Coupled with the advent of much heavier kettlebells now in commercial production (64kg kettlebells are now easily available in Europe and 200lb kettlebells can be purchased in the USA), it now seems likely that kettlebells are a useful tool for any strength and conditioning professional.

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History of kettlebells

Much has been written about the history of kettlebells, although primary sources are difficult to locate. Secondary sources ascribe their origin to Russia and suggest that they have a long heritage of being used there in strength and conditioning, particularly in the military (Tsatsouline, 2006). Whatever their precise heritage, kettlebells were likely originally developed as set weights for weighing goods such as crops for sale. This would explain why they were cast in set weight increments. These increments are called poods, where 1 pood is equal to around 16kg (36lbs). Today, many commercial providers cast kettlebells in 4kg increments, ranging from 4kg through to >60kg.

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Program design

Various reviews have been performed of kettlebell training with suggestions for how programs might be developed in order to meet a range of goals in strength training for athletes (Chiu, 2007; Campbell and Otto, 2013; Beardsley and Contreras, 2014; Jonen and Netterville, 2014), functional training for the general population (Liebenson, 2011), and physical therapy (Brumitt et al. 2010). Indeed, studies have reported both clinically relevant improvements in neck, shoulder and low back pain (Jay et al. 2011) as a result of kettlebell training programs, as well as improvements in sports-specific measures such as vertical jumping (Lake et al. 2012b; Otto et al. 2012; Manocchia et al. 2013; Jay et al. 2013). However, specific measurements have not been reported in relation to any validated measures of functional status, or in relation to any other rehabilitative outcomes.

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Technique

Several coaches have presented guidance on how to perform some of the most common kettlebell exercises, including the 1 and 2-hand swing (Harrison et al. 2011), clean and press (Harrison et al. 2011), goblet squat (Harrison et al. 2011), Turkish get up (Liebenson and Shaughness, 2011; Leatherwood et al. 2014) and thruster (Eckert and Snarr, 2014). In general, the 2-hand swing is probably the most commonly-performed kettlebell exercise. In this respect, it is important to note that there are two main variations: the squat kettlebell swing and the hip-hinge kettlebell swing (see review by Matthews and Cohen, 2013). The squat kettlebell swing is thought to place more focus on the quadriceps while the hip-hinge kettlebell swing is thought to place more emphasis on the hamstrings. In addition, a version called the “American swing” is described in the CrossFit Journal, which involves swinging the weight all the way to overhead (Glassman, 2004).

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Safety

Like all resistance training modalities, there are risks associated with kettlebells. In general, the risks associated with resistance training are poorly understood and have not been widely researched. Because of the prevailing direction of ground reaction forces (Lake and Lauder, 2012a) and spinal loads (McGill and Marshall, 2012) differ substantially between kettlebell training and traditional barbell resistance training, it is possible that the risks associated with kettlebells may differ from those associated with more common modalities of resistance training. One case study reported how a patient who used kettlebells for training for a 3 month period developed radial wrist pain. Specifically, the tendons of extensor pollicis brevis and abductor pollicis longus in the wrist were tender and sore. An ultrasound scan revealed that the extensor pollicis brevis had actually split, which was in retrospect attributed to direct impacts during overhead jerks (Karthik et al. 2013). This suggests that care should be taken during kettlebell exercises for the weight not to move and fall against the hand or wrist, particularly during dynamic movements.

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CONCLUSIONS REGARDING KETTLEBELLS

Kettlebells are probably most well-known when used ballistically. The most popular ballistic kettlebell exercise is the swing.

Safety considerations may differ between common kettlebell exercises and traditional resistance training modalities.

EFFECTS ON STRENGTH AND POWER

PURPOSE

To assess the effects of long-term kettlebell training on measures of strength and power.

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EFFECTS ON STRENGTH

Introduction

So far, there have been only a small number of long-term investigations exploring improvements in strength following standard programs of kettlebell training. In general, it appears that training kettlebells may well improve measures of strength. Moreover, some (Otto et al. 2012) but not all (Lake et al. 2012b) studies comparing kettlebell training with traditional training methods have found that traditional training methods are superior for strength gains.

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The following table summarizes those studies that have assessed the ability of kettlebells to produce long-term strength gains.

EFFECTS ON POWER

Introduction

To date, no studies have assessed actual gains in power output following a long-term period of kettlebell training, although several have reported on changes in proxy measurements (e.g. vertical jump or performance in a ballistic resistance training exercise). In general, it appears that training kettlebells do improve vertical jump height or performance in a ballistic resistance training exercise. Moreover, studies comparing kettlebell training with traditional training methods (Otto et al. 2012; Lake et al. 2012b) have found that traditional training methods produce similar improvements as kettlebell training. It seems logical that kettlebell training is more likely to demonstrate greater improvements in high-velocity-low-force movements than in low-velocity-high-force movements, as light weights are typically used in the programs tested. Different results might arise if researchers later choose to investigate programs involving heavier kettlebells.

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The following table summarizes those studies that have assessed the ability of kettlebells to produce long-term power gains, as measured by performance in proxies, such as the vertical jump or ballistic resistance training exercise.

CONCLUSIONS REGARDING KETTLEBELLS

Kettlebell training programs seem to be able to improve strength in (mostly untrained) individuals. They also seem able to improve proxies of power (vertical jump height or ballistic resistance training exercise performance) in similar populations.

EFFECTS ON AEROBIC FITNESS

PURPOSE

To assess the effects of long-term kettlebell training on measures of aerobic fitness (such as maximal aerobic capacity) and to study the short-term physiological response to kettlebell training, by reference to acute heart rate and oxygen usage.

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LONG-TERM EFFECTS ON AEROBIC FITNESS

Introduction

So far, the research investigating long-term changes in aerobic fitness subsequent to kettlebell training programs is limited. This is unfortunate, as kettlebell training is frequently presented as a method of simultaneously creating both muscular and cardiovascular adaptations.

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The following table summarizes those studies that have assessed the ability of kettlebells to produce long-term gains in aerobic fitness.

Kettlebell training involving snatches for 20 minutes, with 15 seconds work and rest intervals, 3 times per week

4 weeks

Kettlebell training improved VO2-max

Kettlebells vs. control =2.3 vs. 0.3 ml/kg/min

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ACUTE EFFECTS ON HEART RATE RESPONSE

Introduction

Since there are so few studies investigating the long-term effects of kettlebell training programs on aerobic fitness, it is valuable to assess their acute physiological effects. Indeed, the American College of Sports Medicine (ACSM) originally recommended that exercise involving >55 – 65% maximum heart rate is sufficient to develop aerobic fitness (Pollock et al. 1998) although it is noted that this guidance was updated more recently and omits direct reference to heart rate (Garber et al. 2011). In this respect, several studies have investigated the acute heart rate response to kettlebell training. In general, these studies have found that the heart rate response was above the levels proposed as necessary to achieve increases in aerobic fitness. This is unsurprising, as similar effects have been reported for resistance training (e.g. Hrubeniuk et al. 2014).

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The following table summarizes those studies that have assessed the ability of kettlebells to produce a short-term heart rate response that might be reflective of long-term gains in aerobic fitness.

ACUTE EFFECTS ON OXYGEN COST

Introduction

Since there are so few studies investigating the long-term effects of kettlebell training programs on aerobic fitness, it is valuable to assess their acute physiological effects. Indeed, the American College of Sports Medicine (ACSM) originally recommended that exercise involving 40 – 50% of VO2-max would be sufficient to develop aerobic fitness (ACSM, 1990; Swain and Franklin, 2002). Later research identified that percentages of VO2-max were likely an unreliable way of assessing comparable intensities of exercise across both trained and untrained individuals and that VO2-max reserve was probably a more useful measure (Swain and Franklin, 2002). Nevertheless, several studies have investigated the oxygen uptake (as a percentage of VO2-max) in response to bouts of kettlebell training. In general, these studies have found that the oxygen uptake was above the minimum levels of VO2-max that were originally proposed as necessary to achieve increases in aerobic fitness.

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The following table summarizes those studies that have assessed the ability of kettlebells to produce short-term oxygen costs that might be reflective of long-term gains in aerobic fitness.

CONCLUSIONS REGARDING KETTLEBELLS

Kettlebells may well be able to increase aerobic fitness, although evidence from long-term trials is currently limited.

BIOMECHANICS OF KETTLEBELL TRAINING

PURPOSE

To assess the biomechanics of common kettlebell exercises in order to establish how they might best transfer to athletic performance.

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BACKGROUND TO KETTLEBELL EXERCISES

Introduction

Types of kettlebell exercise

Kettlebell exercises can be either ballistic or non-ballistic. Given that kettlebell exercises appear to be more useful for developing power than strength, it seems likely that ballistic kettlebell exercises will be more useful to athletes than non-ballistic exercises. The most common types of ballistic kettlebell exercise are the swing and the snatch.

Types of kettlebell swing

There are two main styles of kettlebell swing: the hip-hinge swing and the squat-dominant swing (see review by Matthews and Cohen, 2013). The nature of these two types of swing is thought to differ in respect of the involvement of the lower body muscles. The hip-hinge swing is thought to lead to a similar pattern of muscle recruitment to the deadlift, while the squat-dominant swing is thought to involve a similar pattern of muscle recruitment to the squat (see review by Matthews and Cohen, 2013). Thus, the hip-hinge swing is believed to target primarily the hamstrings and gluteus maximus, while the squat-dominant swing is believed to exercise the quadriceps and gluteus maximus. The hip-hinge swing could therefore be very valuable to strength and conditioning coaches, as they allow training of the hamstrings at fast, more sports-specific speeds.

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ELECTROMYOGRAPHY (EMG) OF KETTLEBELL EXERCISES

Introduction

Electromyography (EMG) is a method used to detect the level of neural drive or voluntary activation in a muscle. Voluntary activation is affected by both the degree of muscle recruitment and the motor unit firing frequency and is closely related to muscle force in unfatigued muscles. Researchers have generally agreed that the EMG activity in a muscle during an exercise in a single training session is indicative of the potential long-term adaptations in that muscle. Therefore, EMG studies represent a valuable way of assessing how kettlebell exercises might best be used in athletic development or for general exercise.

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Hamstrings EMG activity

Introduction

To date, only two studies have reported on the EMG activity of the hamstrings during kettlebell exercises (McGill and Marshall, 2012; Zebis et al. 2013). McGill and Marshall (2012) assessed biceps femoris EMG activity during the squat-style 1-hand kettlebell swing, squat-style kettlebell swing with kime, squat-style kettlebell snatch, racked kettlebell carry, and bottoms-up kettlebell carry. For each exercise, the young, healthy but untrained male subjects used a 16kg kettlebell. Zebis et al. (2013) assessed both biceps femoris and semitendinosus EMG activity during hip-hinge style 2-hand kettlebell swings and a range of other non-kettlebell exercises. For each exercise, the young, healthy and resistance-trained female subjects used either a 12kg or a 16kg kettlebell, depending upon their strength levels. Biceps femoris EMG activity reached 93 ± 31% of maximum voluntary isometric contraction (MVIC) levels while semitendinosus EMG activity reached even higher levels at 115 ± 55% of MVIC. In contrast, McGill and Marshall (2012) found that hamstrings EMG activity was relatively low in the squat-style 1-hand kettlebell swing, squat-style kettlebell swing with kime, squat-style kettlebell snatch (32.6%, 39.7% and 29.8% of MVIC), particularly in comparison with the hip musculature (gluteus maximus and gluteus medius). These differences are likely a result of the type of kettlebell swing used (see review by Matthews and Cohen, 2013).

Regions within the hamstrings

Researchers have observed that the EMG activity of the hamstrings is greatest during kettlebell swings in high degrees of hip flexion whereas the EMG activity is greatest during many other commonly-performed hamstrings exercises (such as the Nordic curl) in low degrees of hip flexion (Zebis et al. 2013). This could mean that the kettlebell swing might lead to regional hypertrophy in different areas from other exercises. Regional hypertrophy is thought to be dependent upon the area in which muscle activity occurs during performance of a given exercise. For example, Wakahara et al. (2012) reported that the EMG activity in certain regions of the triceps during a resistance training workout was associated with regional hypertrophy in those same areas after a long-term program. Additionally, different exercises targeting the same muscle are believed to lead to EMG activity in different regions of that muscle (Mendiguchia et al. 2013). Practically speaking, training the hamstrings using kettlebell swings could provide a valuable complement to other exercises, by emphasising regional development in other parts of the muscle and thereby ensuring overall greater muscular hypertrophy without localized weaknesses.

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Gluteus maximus EMG activity

Introduction

To date, only one study has reported on the EMG activity of the gluteus maximus during kettlebell exercises (McGill and Marshall, 2012). McGill and Marshall (2012) assessed gluteus maximus EMG activity during the squat-style 1-hand kettlebell swing, kettlebell swing with kime, racked kettlebell carry, and bottoms-up kettlebell carry. For each exercise, the young, healthy but untrained male subjects used a 16kg kettlebell. McGill and Marshall (2012) found that gluteus maximus EMG activity was relatively high in the squat-style 1-hand kettlebell swing, squat-style kettlebell swing with kime, squat-style kettlebell snatch (76.1%, 82.8% and 58.1% of MVIC), compared with other muscles and the swing displayed greater EMG activity to the snatch.

Regions within the gluteus maximus

Kettlebell swings appear to involve peak EMG gluteus maximus activity late in the swing cycle, which is close to full hip extension (McGill and Marshall, 2012). This is an important finding, as most commonly-used resistance training exercises involve greater gluteus maximus EMG activity in peak hip flexion (Caterisano et al. 2002; Escamilla et al. 2002), most likely because of the greater hip extension moments at greater depths (Bryanton et al. 2012). Practically speaking, training the gluteus maximus using kettlebell swings could therefore provide a valuable complement to other exercises, by emphasising regional development in other parts of the muscle and thereby ensuring overall greater muscular hypertrophy without localized weaknesses. In addition, it is known that the gluteus maximus achieves greater EMG activity when it contracts in full hip extension compared to when it contracts in greater degrees of hip flexion (Worrell et al. 2001). Therefore, the kettlebell swing could be a superior exercise for training the gluteus maximus at high velocities than the jump squat, which involves a peak contraction in a greater degree of hip flexion.

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KINETICS OF KETTLEBELL EXERCISES

Introduction

There have been several assessments of the kinetics (forces, loads and moments) of kettlebell exercises. In general, there have been two main areas of research. Firstly, some groups of researchers have been interested in the implications of kettlebell exercises for spinal loading and the potential consequences for low back pain, on the basis of anecdotal reports that kettlebell training can be pain-relieving in some cases but painful in others (McGill and Marshall, 2012). In this respect it is interesting that one study has reported clinically relevant improvements in neck, shoulder and low back pain following a program of kettlebell training (Jay et al. 2011). Secondly, other groups of researchers have explored whether kettlebell exercises produce similar force, power, and impulse to conventional exercises and whether the direction of force (vertical vs. horizontal) differs (Lake and Lauder, 2012b; Lake et al. 2014). Direction of force is particularly valuable for strength and conditioning coaches working with sprinters, as horizontal force production is now widely recognised as being very important for sprint running performance (see review by Randell et al. 2010).

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Spinal loads

The nature of spinal loads in common kettlebell swings appears to differ substantially from that reported in more traditional resistance training exercises, primarily because of the great difference between shear and compressive loads. McGill and Marshall (2012) reported that compressive loads were 3,195N at the bottom of the swing, while shear loads were just 461N. Compressive and shear spinal loads for the kettlebell snatch were found to be very similar to those reported in the swing (McGill and Marshall, 2012).

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Force and power

Introduction

Athletes require high levels of strength in order to perform well. However, they must also be able to display this force quickly. Consequently, power is often thought to be more important than strength for determining athletic performance because of its ability to indicate the ability to produce force at sports-specific speeds. The most common ballistic resistance training exercise for developing power in athletes is the barbell jump squat. Jump squats require athletes to develop substantial amounts of ground reaction force very quickly in order to leave the ground holding the barbell. This high velocity combined with large force is associated with high power outputs. Strength and conditioning coaches therefore make regular use of barbell jump squats in their programs.

Kettlebell swing: power outputs

Comparing the power outputs of kettlebell swings with jump squats is one way to assess the usefulness of kettlebell training in developing athletes. Lake and Lauder (2012a) investigated power outputs during hip-hinge kettlebell swings with loads from 16kg to 32kg and compared them with jump squats with loads from 0 – 60% of 1RM. Power outputs during jump squats were maximized at no load as expected, while power outputs during kettlebell swings were maximized at 32kg. Comparing the power outputs of kettlebell swings with jump squats identified no significant differences between the two types of exercise, although the jump squat tended to be greater (3,281 ± 970 vs. 3,468 ± 678W). It was therefore suggested that kettlebell swings might be appropriate for inclusion into a power-based program.

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Kettlebell swing: optimal load for power

Most studies report that the load that maximises power output is typically no load during standard barbell jump squats (e.g. Cormie et al. 2007). This has led to the Maximum Dynamic Output (MDO) hypothesis, which proposes that suggests that the lower body muscles have evolved to produce maximal power output in the vertical jump with no load (i.e. bodyweight) rather than with heavier loads (Nuzzo et al. 2010). Interestingly, shifting the position of the center of mass by performing a hex-bar deadlift jump seems to increase this load to around 20% of 1RM (Swinton et al. 2012; Turner et al. 2014). Additionally, power output using hex-bar deadlift jumps seems to be greater than during standard barbell jump squats, where each uses the optimal load for power output (Swinton et al. 2012). Exactly why the hex-bar deadlift jumps maximize power outputs at loads greater than bodyweight is unclear but may relate to joint angle positions during the movement. Obviously, kettlebell swings are unlikely to produce maximum power outputs with no load. Nevertheless, the exact load that maximises power output is as yet unclear. Lake and Lauder (2012a) found that heavier kettlebells (32kg) produced greater power outputs than lighter kettlebells (16kg) during the hip-hinge kettlebell swing exercise (3,281 ± 970 vs. 2,371 ± 708W). Whether even heavier kettlebells would involve even greater power outputs (and where the maximum load for power occurs) is unknown. Research in this area will likely be hampered by the lack of a normal standard against which to measure the relative load of the kettlebell (as percentage of 1RM is not feasible).

Kettlebell swing: impulse

Some reviewers have suggested that power outputs are not as useful as other acute mechanical variables for determining the properties of an exercise (Knudson, 2009). In this respect, it has been proposed that impulse is a superior measurement, as it provides information about both the magnitude and duration of the applied force (Knudson, 2009; Lake and Lauder, 2012a). Although impulses have not been extensively researched in relation to exercise transfer to sports performance, it is important to note that impulse is what produces vector changes in linear momentum. Sprint momentum in particular is believed to have the ability to differentiate between athletes of different abilities (Baker and Newton, 2008; Barr et al. 2014). Comparing impulses during kettlebell swings and jump squats is therefore another way to assess the usefulness of kettlebell training in developing athletes. Lake and Lauder (2012a) investigated impulses during hip-hinge kettlebell swings with loads from 16kg to 32kg and compared them with jump squats with loads from 0 – 60% of 1RM. Impulses during jump squats were maximized at 40% of 1RM, while impulses during kettlebell swings were maximized at 32kg. Comparing the maximum impulse during kettlebell swings with the maximum impulse during jump squats identified that the kettlebell swing was superior (276 ± 45 vs. 231 ± 47Ns). This implies that the kettlebell swing involved a change in momentum that was greater than in the jump squat, which could have sports-specific relevance. Whether even heavier kettlebells would involve the production of even greater impulses is unknown.

Kettlebell swing: direction of force application

The horizontal and vertical components of ground reaction forces during kettlebell swings are different to that during jump squats. Lake and Lauder (2012a) observed that kettlebell swings had a much higher proportion of horizontal forces than jump squats. This might be because the kettlebell is actively projected horizontally forwards by hip extension at the start of the swing. Hip-hinge kettlebell swings may therefore have applications in certain sporting movements that involve hip extension in order to create horizontal propulsion, such as sprint running (see review by Randell et al. 2010). Hip-hinge kettlebell swings may therefore be useful for developing sprint running performance in athletes.

Kettlebell swing: comparison with snatch

In order to assess whether the kettlebell snatch or kettlebell swing are more well-suited for use by strength and conditioning professionals, Lake et al. (2014) compared the mechanical demands of each exercise. They found that the two exercises were largely similar except in relation to the proportion of horizontal and vertical mechanical demands. Specifically, they noted that the kettlebell swing involved significantly greater horizontal work, horizontal power, horizontal braking and propulsive impulses, and horizontal braking and propulsive ground reaction forces than the kettlebell snatch. This indicates that the kettlebell swing may be superior for applications requiring horizontal force production at sports-specific velocities, such as sprint running.

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CONCLUSIONS REGARDING KETTLEBELLS

Kettlebells seem to involve peak muscle activity at different points in the joint range of motion to other exercises, which may make them useful complementary training tools.

Kettlebell swings display several features that may make them valuable for improving sprint running ability: a medial hamstring focus, peak activation of the gluteus maximus in hip extension, and greater horizontal force production.

REFERENCES

American College of Sports Medicine. (1990). Position stand on the recommended quantity and quality of exercise for developing and maintaining cardiorespiratory muscular fitness in healthy adults. Medicine & Science in Sports & Exercise, 22, 265-274.